authors: initial release

* Introduces SQLAlchemy models for Publication, Author, Cluster and Signature.
* Adds Citation association table.

Signed-off-by: Mateusz Susik <mateusz.susik@cern.ch>
authors: initial skeleton for module

Signed-off-by: Gilles Louppe <g.louppe@cern.ch>
authors: skeleton test file
authors: JSON field in Signature empty by default.
tests
authors: record property and __repr__ for models